“Pressure sensors” in the sense used herein includes absolute pressure sensors and relative pressure sensors, the first of which, measures, for a medium being measured, absolute pressure compared with vacuum, and the second, the difference between pressure in a medium being measured and the current atmospheric pressure. A pressure sensor includes, in general, a pressure measuring cell comprised of a base plate and a measuring membrane, or diaphragm, with a pressure chamber being formed between the measuring membrane and the base plate. The pressure-dependent deformation of the measuring membrane is a measure for the pressure, and such is converted in suitable manner into an electrical quantity, or primary signal. For conditioning the primary signal, or electrical quantity, usually there is an electronic circuit, for example a hybrid circuit, arranged preferably in the immediate vicinity of the primary signal source, for example on the rear face of the base plate, the face not containing the pressure chamber. The electronic circuit, it is true, is arranged in a sensor housing, for protecting it from dirt, etc., but is still exposed to the air of the environment. In such case, especially under realistic operating conditions fluctuating air humidity proves to be a difficultly manageable source of malfunctions, since it can lead to, among others, changes in the properties of the components of the electric circuit and even of the connecting lines and, consequently, to corruptions of the measurement signals. These corruptions are, it is true, small and acceptable for standard applications, but, in the case of precision sensors, something must be done about them. Thus, there are efforts to minimize the influences of moisture by encapsulation of the electric circuit. Unpublished International Patent Application No. PCT/EP02/14787 of the present assignee discloses, to this end, a ceramic, capacitive, relative pressure sensor having a ceramic pot connected to the rear face of the pressure sensor, in order to form a chamber in which the hybrid circuit enclosed in the chamber is protected from the influences of moisture. This concept is, however, capable of improvement for the following reasons.
Even though the pot is held in place by adhesive, moisture can still get inside the pot, because organic materials do not offer satisfactory long-term protection, this being indicated, for example, in a report entitled “Hermeticity of Polymeric Lid Sealants” by R. K. Traeger, New Initiatives, Sandia Laboratories, Albuquerque, N.Mex., USA. Additionally, the pot in the case of the pressure sensor of the above application extends over the entire rear face of the base plate. Therefore, the rear, axial support of the pressure measuring cell against the process pressure must occur through the pot and especially through the joint between pot and base plate. Sufficiently strong, interfacial, brazed, or hard-solder, joints cannot be implemented in the presence of the enclosed hybrid circuit, because the temperatures required for such would be too high for the hybrid circuit. The use of soft solder for joining the pot with the base plate can lead to hysteresis phenomena, since the joint is susceptible to plastic deformation due to pressure shocks and temperature fluctuations.